The role of singlet oxygen in drug-induced photosensitization has been investigated by means of electron spin resonance. Irradiation of anthracene, 9-aminoacridine, indoxole, chlortetracycline and declomycin with visible light produced high concentrations of singlet oxygen. By contrast, the sulfonamide drugs chlorthiazide, tolbutamide and sulfadiazine did not generate singlet ozygen on light irradiation. These studies indicate that singlet oxygen may be involved in the photosensitization caused by certain drugs.